There are new proposals coming for allow streaming HTML into an existing document. See whatwg/html#11542 and whatwg/html#2142 (comment).

For this to work, it would need to come along with sanitation support.

To connect the dots, trusted-types would need some form of sanitizing an HTML stream on the go, probably using a TransformStream.

This can have the following shape:policy.createHTMLTransformStream(), where the source of the untrusted HTML would be piped to the result's writable and the readable would be piped to the element receiving the markup.